Where I am at geographically, I prefer morning rides for the following reasons, in no particular order:
1) less wind;
2) fewer airborne bugs on the routes I use;
3) fewer joggers/cyclists/cars/folks on the routes I use;
4) it tends to be cooler (mid-60's this morning, how great is that).
However my work schedule is changing effective next week, and I will be forced to cease morning rides except on the weekends. So I'll probably go back to afternoons when the weather cools off, but for now, late afternoons/early evenings. But I always have to get my permission slip signed first by my better half... that's another reasons crack-of-dawn rides were great for me, as everyone else in the family was still in bed, so it caused minimal interference with the daily routines.
Whatever works...